1. Field of the Invention
This invention relates to a control device for controlling servo motors, and more particularly to a control device for controlling and driving a plurality of servo motors to move an object to a predetermined position within a mechanical coordinate system.
2. Discussion of the Related Art
Machine tools conventionally include a tool for tooling a workpiece and a table for supporting the workpiece. Machine tools are conventionally equipped with a servo motor control device for driving and controlling a plurality of servo motors to move an object such as a tool or a table in any direction within X-, Y- and Z-axes that constitute a mechanical coordinate system. When the servo motors are driven and controlled by the servo motor control device, the object is moved to any desired position within the mechanical coordinate system.
Typically, the table of the machine is connected to a first servo motor that drives the table along the X-axis and to a second servo motor that drives the table along the Y-axis. Under control of the servo motor control device for controlling both the first and second servo motors, the table follows composite movements in the X and Y directions.
A previous servo motor control device comprises an interpolating section and a position control section for determining and attempting to move an object along a target locus. The interpolating section establishes a series of target positions of the object on each of the coordinate axes for a series of time intervals, each time interval having a predetermined minimum unit time, for example, 10 msecs. The position control section controls the rotational speeds of the individual servo motors, using the same prescribed position control gain for each of the coordinate axes, and based on a deviation between the target position of the object and the current position thereof.
However, a number of disadvantages have been encountered in connection with the above-described previous servo motor control device. One disadvantage is a time lag involved in transmitting, on a time-sharing basis, the target position of the object from the interpolating section to a position controller for each of the coordinate axes. Another disadvantage is that the individual position controllers for each coordinate axis have their own intrinsic delays in responding to incoming signals. Consequently, the object cannot be moved accurately to target positions if the same position control gain is used to control the servo motors on each axis.